1. Field
The invention is in the field of ion exchange of various metallic and non-metallic anions. The term "ion exchange" is considered broadly to include extraction of ions from solutions by the use of both liquid and solid extractants.
2. Prior Art
Ion exchange is well known and widely used in hydrometallurgy and water purification. Liquid ion exchange, generally known as solvent extraction, carried out by various organic liquid extractants, with or without diluents or modifiers, has been used in hydrometallurgy for the recovery of uranium, copper, and other metals. Natural zeolites have long been used as a solid ion exchange material for water softening, but have been largely replaced by synthetic resins which are also widely used for waste water purification and in hydrometallurgy for the separation of metallic ions. The use of ion exchange resins is a form of ion exchange that provides a solid substrate. Such a substrate may have chemically combined or attached moieties or groups active in the ion exchange process or the substrate may function merely as a carrier for an active extractant such as inert resins impregnated with liquid extractants. The use of activated carbon, as in the recovery of gold from solution, can be and is here regarded also as a form of ion exchange involving a solid substrate and is included within the scope of the invention when treated with a modified amine extractant.
Ion exchange in its various forms has received considerable attention in recent years, and is being widely used in particular instances, especially in hydrometallurgy. However, due to limitations inherent in the nature of many extractants, there has been a long felt need to widen the scope and improve the technology of various ion exchange systems. Such a situation exists in the case of amines capable of protonation, i.e. weak base amines. Their extractive properties at low pH values have been known, but their inability to act as extractants at high pH values has limited their usefulness.
The amines capable of protonation which have been used on an industrial scale as metallic and non-metallic anion extractants are generally primary, secondary, and tertiary amines, especially alkyl amines. Instances of such use of secondary and tertiary alkyl amines are as follows:
TABLE A ______________________________________ Aqueous Feed Amine Metal Solution Extractant ______________________________________ Molybdenum Acid Sulfate Tertiary Tungsten Acid Sulfate Secondary Uranium Acid Sulfate Tertiary Vanadium Acid Chloride Tertiary ______________________________________
It is important to note that industrial use of these amine extractants has been limited to acidic solutions. This is because amines capable of protonation are such weak bases that they can only effectively act as extractants in acidic and neutral solutions.
In the laboratory, primary, secondary, and tertiary alkyl amines have been used for the recovery of anions from solution. A number of metals exist as anionic complexes. However, extraction of such metals using amines as ion exchange extractants is generally limited to pH values less than 7. This can be seen from the following compilation based upon pH.sub.50 extraction with 0.05 M amine/xylene solutions:
TABLE B __________________________________________________________________________ Amine Basicity, pH.sub.50 Gold Iron Amine Cyano- Cyano- Molybdenum Chrominum Vanadium Extractant anion anion Oxyanion Oxyanion Oxyanion __________________________________________________________________________ Primary 6.55 5.50 5.85 5.90 6.96 (Primene JM-T) Secondary 7.15 5.70 5.80 6.35 6.90 (Adogen 283) Tertiary 5.66 -- 5.08 4.80 5.18 (Alamine 336) __________________________________________________________________________
Primene JM-T, produced by Rohm and Haas, is a primary alkyl amine having 18 to 22 carbon atoms in the alkyl group. Adogen 283, produced by Sherex, is a secondary amine having the formula R.sub.2 NH wherein R is an alkyl group of 13 carbon atoms. Alamine 336, produced by Henkel, is a tertiary amine having the formula R.sub.3 N wherein R is an alkyl group having eight to ten carbon atoms.
As is well known, the pH.sub.50 of a given extraction is that pH at which fifty percent of the desired anion has been extracted from the solution. It can be considered to be a measure of the amine's strength, i.e. ability to extract, or an indication of basicity. A higher value of pH.sub.50 is indicative of an increase in base strength or basicity. The pH.sub.50 is used by those skilled in the art as an indication of the pH region in which extraction can be accomplished.
The recovery of gold from gold-cyanide leach solutions is of particular interest, as such recovery has heretofore not been possible using solvent extraction techniques. This is because a suitable extractant for solvent extraction from alkaline cyanide solutions has not heretofore been known. Gold has generally been recovered from cyanide leach solutions through processes using carbon in pulp, carbon in columns, or through the Merrill-Crowe process. Solvent extraction, if perfected, has certain advantages over such techniques.
As is shown above, the use of primary, secondary, and tertiary alkyl amines as extractants has previously required acidic conditions. Since lowering the pH of a gold-cyanide leach solution to below 9.3 may cause the evolution of toxic cyanide gas, such an extraction process using these amines has not previously been practical.
Tributylphosphate (TBP) and other organic phosphorous oxides have been used in industrial plants as either extractants or modifiers. They have been used to extract metallic anions from acidic solutions, or to modify acidic extractants in the recovery of metallic cations. However, so far as we are aware, neither IBP nor any other organic phosphorous oxide has ever been effectively employed as a modifier of an amine, or of an amine group compound, such as analine, amidine, or guanidine, for industrial solvent extraction purposes. This is consistent with published experimental results which show that adding TBP or other organic phosphorous oxides to tertiary amines causes only a slight enhancement of the extraction of certain metallic anions. ("Recent Developments and New Combinations of Extractants in Synergic Processes" by G. Duyckaert and J. F. Desreux, Canadian Institute of Metallurgy, Special Volume 21, 1979, pages 73, 82.) Such a slight enhancement has not encouraged the investigation of the possible use of organic phosphorous oxides as amine modifiers in an ion exchange system.